Toyota Corolla, 135,000 miles, plugs replaced @ 100,000. CEL came on a while ago P0351, ignition coil cyl 1, after a rough start and idle. Turned the car off restarted and the car ran fine but CEL still on. Cleared code. Swapped coil between 1&2, few weeks later same code and rough start Again, turned car off restart and ran fine. Cleared code. A few weeks later same thing, swapped plug bet 1 & 2… Today same symptom and same code. It may be my imagination but the car may be running a little rough, but I may also be looking for problems since the 1st CEL. Mileage has not changed. The CEL is always for coil 1. Any ideas on what to check next? Harness or fuel injection? Thanks much!
I would check the harness carefully, with a multimeter, while carefully wiggling the wiring harness?
any spread terminals?
does the connector audibly click, when you plug it in?
reference voltages and ground good?
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There were wiring harness problems with the early built vehicles. Take a look at this bulletin, see if your VIN is before the production change;
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I thought I would have to check the wiring harness next, my VIN is a later # so I have one that did not have a known problem. I will start tracking this down later in the week.
Thanks @db4690 & @Nevada_545 for the info
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Have not had a chance to check the harness yet, but I have not found a harness on Rockauto etc.I have found an ignition coil connector. Is that what I should be looking for? Not ready to buy it yet, just starting to line up all my options.
I think I solved the problem. My wife and I both had smelled unburned fuel more than once. I thought the misfire was causing the unburned fuel. But I smelled unburned fuel even without a misfire. So I ordered up some fuel injectors and replaced them. So far so good. The injector on #1 must have been leaking, during start up there was too much fuel and it caused a misfire. (that’s my story and I’m sticking with it 
)
Now on to the injector saga. I ordered them off of a popular web site many of us use. I picked the part because all the o rings were included. I installed them and the engine started up nicely, however when I looked at them I had gas spewing all over. Took it apart, checked for dirt mis alignment etc and re seated them. Started the car up and had gas spewing all over again. At this point I am thinking I could have had my local shop do it, more money but less aggravation -but that is another post. I took it apart and checked to make sure the right injectors were sent, I have had incorrect parts shipped before. When I held the new to old they were the same except for the o ring on the top-input side. The original were red and tight, the new ones were brown and loose, loose to the point they had deformed a bit, rolled out of their slot. There was no way they could hold a seal. I could have messed up one o-ring but not all 4. I swapped o rings reinstalled and no more leaking gas. The old red o rings were snug in the slot.
The top of my engine is nice and clean now. And so far no more misfires on start up.
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Sound like you have it under control there OP. When I replaced fuel injector O-rings on my Rabbit they recommended I soak the O-rings in gasoline for an hour or two prior to the install, said to help the o-rings seal better. I think the gasoline soak plumped them up a little. It had the adverse effect of making them a little more difficult to install tho .
Do you have a theory why an injector problem would cause a coil diagnostic code?
The last time I did injectors, the factory service information recommended lubing the seals with fresh engine oil
Worked just fine
I find i kind of hard to believe VW would recommend soaking fuel injector o-rings in gasoline for an hour
Wouldn’t that cause the seals to swell?
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doing work on my old Pathfinder, I had a theory my injectors were at fault and I removed/cleaned/reinstalled them. what dealer’ repair manual was stressing in bold font is that rail side and manifold side O-rings are not interchangeable, and indeed the rail side was visually thicker.
They did also mention using motor oil to make sure O-rings are not damaged as injectors are inserted into the rail.
For rail-side I went to dealer to get O-rings as I was afraid of leaks if aftermarket would be used, it was really not that expensive, something like $10 for set of 6.
By the dealer repair manual I had to replace snap-rings too, but I skipped that part.
Assembled and got it working and no leaks from the first try.
I did have a light coating of oil on the o-rings. The first leak probably washed it all off though. I did not measure the o rings, but visually the brown replacements seemed a bit bigger and did not seat tightly in the groove. The old red ones were nice and snug. My working theory is the leaking injector caused a rich condition/mis fire and the mis fire was reported as PO351.
Don’t have a VW factory service manual at hand, but this practice seems to continue in the VW community, for example, this VW forum post …
"_Re: How difficult is it to replace 16V injector o rings (2drJettaVR6) _
_11-06-2002 03:59 PM #12 _
Easy, use one of those pry bars used to pry wood and nails. Support the injector from the bottom with your hand and pry it out. to insert the new Orings, let them soak in fuel for an hour or so, , get the needle nosed pliers, slide the Oringon the pliers, open the pliers enough to have the injector between the arms and with a little fuel, slide the Oring over to the injector. "
And yes, it does cause them to swell a little. That’s the idea, to make the seal firmly. And the gasoline on the surface provides some lube to get them to seat. I expect soaking in oil would work too, but might make them slippery and therefore easier to be pulled out by accident, or blown out were the engine to backfire into the intake manifold. That was more of a conern with the mechanical fuel injection system, because there was no fuel rail per se, each of the injectors were connected to the fuel distributor with individual hoses, so there was no force besides the o ring holding them in place. The fuel rail method has the fuel rail providing some mechanical support to hold them in place.
Seems dubious to me. That 351 code means the engine computer is not seeing the expected electrical signal pulse from the coil circuit that the coil is has successfully fired. hmm … well I suppose if you had a really rich condition, so rich the the spark plug electrodes got shorted out with fuel, it might fool the computer into thinking the coil was bad. I don’t think so though, I think you may have another problem lurking there.